Interdiffusion and Atomic Mobility for Face-Centered-Cubic Co-Al Alloys
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widely used in aerospace and power industries. Increasing combustion gas temperature is effective for improving the thermal efficiency of gas turbines and, hence, is helpful in reducing the CO2 emissions and stopping global warming. Development of new high-temperature materials is essential for realizing higher operating temperatures. Recently, a new type of cobalt-based alloy, with higher strength and compressibility than the conventional Ni-base superalloys at elevated temperatures, was considered to have potential as a next-generation high-temperature material.[1–4] Just as in the Ni-base superalloys, the new type of Co-base alloys sustain superior temperature capabilities, in large part because the microstructure following heat treatment consists of the face-centered-cubic (fcc) c matrix and the ordered fcc c¢ precipitate. The c¢ precipitate is greatly responsible for the elevated-temperature strength of the alloys and resistance to creep deformation. Its morphology and composition are largely dependent on the diffusion interaction between the c¢ and c phases. The essential solute in the Co-base superalloys is aluminum; therefore, knowledge of diffusion in the Co-Al base system is of critical importance in exploring the new type of Co-base high-temperature Y.-W. CUI, Staff Researcher, is with Computational Alloy Design Group, IMDEA Materials Institute, Madrid 28040, Spain. Contact e-mail: [email protected] B. TANG, Doctoral Candidate, is with the State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, People’s Republic of China. R. KATO, Graduate Student, R. KAINUMA and K. ISHIDA, Professor, are with the Department of Materials Science, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan. Manuscript submitted April 11, 2011. Article published online June 10, 2011 2542—VOLUME 42A, SEPTEMBER 2011
alloys. However, existing diffusion data are very limited for the Co-Al alloys,[5,6] and no atomic mobility data have been reported so far. This article presents interdiffusion studies carried out over a wide temperature range from 1173 K to 1573 K (900 C to 1300 C) with solidstate diffusion couples. The interdiffusion coefficients were evaluated and then employed to assess the atomic mobility for the fcc Co-Al alloys, which could enable much of the diffusion process to be predicted. Binary alloys with compositions of Co-10 at. pct Al and Co-13 at. pct Al were prepared from electrolytic cobalt (99.9 pct) and electrolytic aluminum (99.7 pct) by arc melting under an argon atmosphere. Arc melting was repeated several times to attain a homogeneous composition. Small pieces of specimens cut from the alloy ingots were solid-solution treated under vacuum in quartz capsules at 1573 K (1300 C) for 24 hours followed by water quenching. The solution treatment resulted in the alloys with mean grain size larger than 1 mm such that the effect of grain boundary diffusion could be ignored. Small diffusion disks were prepared by polishing one surface of the specimen to mirrorlike
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